The long-term objective of this proposal is to understand the pathophysiology of autosomal dominant polycystic kidney disease (ADPKD) as a basis for rational therapy, including gene therapy, of the disease. Recent studies have shed considerable light on the genetic basis of ADPKD. Mutations in PKD1, a gene on chromosome 16, account for 90% of cases. The DNA sequence of PKD1 has just been determined. PKD1 encodes a 4,304 amino- acid cell-membrane protein, "polycystin", that has a series of five adhesive extracellular domains, several membrane-spanning segments and a shorter cytoplasmic tail. These data suggest that ADPKD results from a failure of cell-cell or cell-matrix interactions involving polycystin and its ligand(s) such that normal tubular differentiation does not occur or is not maintained. One of the most powerful new approaches for the study of disease pathogenesis is targeted gene mutation. Standard transgenic technology results in insertion of the transgene at quasi-random sites in the genome so that the transgene and endogenous genes are simultaneously expressed. Gene targeting, by contrast, allows mutated sequence to be substituted for the allelic endogenous sequence so that the mutation can be studied in the context of the natural chromosomal (regulatory) environment. The major goal of this proposal is to use these techniques to create and analyze two mouse models of ADPKD, each containing a mutation that mimics a mutation found in man. These models have significant advantages over the study of pathogenesis in man. First, mouse tissues are available from all organs from embryonic development to adult life. Such samples will be used to study the spatial and temporal distribution of polycystin. Second. the mouse model will be used to study the effects of modifier genes, DNA sequences that alter the severity of a disease. These studies can then be used to guide discovery of modifier effects in human kindred. a task which is almost impossible by conventional human genetic linkage analysis alone. Finally, in the longer term, mouse models can be used to develop new therapies and evaluate potential therapies for ADPKD. These studies are much harder to perform in ADPKD patients because the gradual progression of the disease leads to over long-term studies as well as ethical and safety concerns.